pop up telling me a build item has been removed because I don't own it

In my experience when I get that message its from one of those limited time events, like the chair or the grim phone thing. Have you seen anything missing from your catalog? 

I just got this pc recently, and I haven't even done much actual game play, mostly just building. So I wouldn't have even collected any limited time items. I went through my game library and everything is still there, so it's not like the add ons that went away when I stopped having the digital deluxe version (back in 2023). When you say my catalog, do you mean my build/buy catalog? I have almost every add on pack, I would never notice if one specific thing went missing 😅

I have had this happen to me as well and I attribute it to one of three things since I own all content available for purchase.

1. Event items that I did not earn. An example would be the Reaper or Blast From the Past events. This would apply to lots I downloaded from the gallery.
2. Sometimes the EA launcher will say if I sign up for emails I will get a special item. Again this would apply to lots I downloaded and someone put these things in the house.
3.  I have absolutely seen this message happen because of mods I had removed when I was still using them. A good example of that is an elevator mod that I used. The elevators actually functioned like portals so I used them in my larger houses to help sims get around faster. When I removed all of my mods, I would get this message when loading every household because it was letting me know that the the elevators had been removed even though the message doesn't help a person understand "what" was removed. 

These three things are what make sense to me and so I have never considered it a problem. Puzzleaddict makes a good point about the missing door too as I remember seeing that post recently. At some point we would notice a door missing hehe. 

puzzlezaddict​  Thank you! You nailed it, it was that door. I had briefly scanned the build but I missed that a bedroom on the second floor was missing it's access to the balcony. It's been so long since I opened this build that I might not have ever figured it out lol

Nightgypsy​ Thank you for taking the time to give me such details ideas of the problem! I thought it was my mods at first too, but I don't anything as fancy as an elevator mod lol. Just T.O.O.L. and better build buy, and when I removed them the problem persisted. The door was the issue, I didn't notice it the first scan of the build,  but now that I was looking for it I realized it was gone

Lol, I did say we would notice a door missing eventually and I am glad it was as easy as that for you. It was just the first thing that crossed my mind about the mods when you said you hadn't played the save in a while. And you are welcome. I know I hate being frustrated and like to help if I can and sometimes it is just the little things. Take care and have fun!